Add option to not print help if no arguments are given

[MiroPalmu]

Let's say we have program foo which parses command line options with:

argz::parse(about, opts, argc, argv);

and it is run with

./foo

help message is printed due to check in argz.hpp:138

This PR adds new parse function that does not print help if there is no options:

argz::parse_when_no_options_is_possible(about, opts, argc, argv);

So now ./foo will not print help.

parse_when_no_options_is_possible is bad name and could be improved.

[stephenberry]

@MiroPalmu
So, I think this is a great feature, but I'd rather just always call the same parse function in all cases. So, I added print_help_when_no_options to the about struct, which provides an easy way to add this behavior. Do you have any concerns with this?

[stephenberry]

I'm just merging this with my change, but let me know if you'd prefer a different solution.

[MiroPalmu]

Oh that is much cleaner thanks!